A number of proposals and attempts are made to develop surface acoustic wave devices as a propagation medium for a surface acoustic wave which travels along the surface of a piezoelectric substrate. This is because the traveling velocity of a surface acoustic wave is only about 10.sup.-5 times that of electromagnetic waves, thus allowing an extreme reduction in size of respective circuit elements and integration of a large number of circuit elements including surface acoustic wave devices on a single substrate. The second reason for such enthusiasm for surface acoustic wave devices is that a signal can be readily picked up from any desired point of the propagation path because a surface acoustic wave travels near the surface of a solid. The third reason is that since the energy is concentrated along the surface of a solid, a surface acoustic wave device may be readily disposed for interaction with light, the carrier in a semiconductor or the like, and may effect a non-linear action due to the highly concentrated energy. The fourth reason is that a surface acoustic wave device may be simultaneously produced on a single substrate together with other circuit elements, thus cooperating with these circuit elements to provide an IC having a unique capablility.
FIGS. 1 and 2 show prior art surface acoustic wave devices. A semiconductor substrate 1 made of silicon or similar material is provided with a piezoelectric layer 2 or 3 made of zinc oxide or of aluminum nitride. Interdigital electrode transducers 4 and 5 are provided on the zinc oxide layer 2 or on the aluminum nitride layer 3. One of the transducers, the transducer 4, for example, is an input electrode, and the other transducer 5 is an output electrode.
A surface acoustic wave, which is excited and converted from an electric signal by the input electrode 4, travels along the surface of the zinc oxide layer 2 or of the aluminum nitride layer 3, and is picked up from the output electrode 5.
Comparing the two devices of FIGS. 1 and 2 with each other by propagating a Rayleigh wave as a surface acoustic wave, the device of FIG. 2 provides a phase velocity 5000 m/s-5500 m/s, whereas the device of FIG. 1 provides 2700 m/s-5100 m/s. This shows that the device of FIG. 2 provides a higher phase velocity and therefore is suitable for use in a high frequency range.
However, the device of FIG. 2 is inferior to that of FIG. 1 in electromechanical coupling coefficient, which is one of the important factors permitting such a surface acoustic wave device to operate with an appreciable efficiency.